is apple filling up the storage on older devices to force you to get a new one?

I have a couple of older iPads that aren't able to run most apps anymore so I just run one security app and use the iPads to view a security camera. this has worked for the Last 5 Years. about a year ago they started to tell me that the storage was full. so I reset them and within a couple months it said the storage was full again. literally went from over 20G to full in 2 months. 

Both iPads were then reset and both filled in 2 months again. so I reset them, deleted all apps that were deletable, unsynced all cloud services and turned off every setting on both iPads. Again, both iPads starting with over 20 gigs of free space filled within a couple months. I have this app on a newer iphone, and new ipad, and two Androids devices, none of which have this issue. I would hate to see my older iPads hit the landfill when they work just fine as a spot monitor for security systems. any ideas of what's going on?